There is known an acoustic wave device configured so that an electrode such as an interdigital transducer IDT is formed on a piezoelectric substrate. A typical example of such a device is a surface acoustic wave (SAW) device. The electromechanical coupling coefficient, which is one of the characteristics of the acoustic wave device, depends on the material and orientation of the crystal of the piezoelectric substrate, and has a relationship with the fractional bandwidth of the acoustic wave device. There is known an art of adjusting the electromechanical coupling coefficient by changing the thickness of a dielectric layer such as a silicon oxide layer or aluminum oxide layer formed below the IDT electrode (see Japanese Patent Application Publication No. 2008-67289: Document 1).